The present development relates to a process for enrobing active catalytic materials with a protective coating to form pastilles, and to an apparatus for making the pastilles. The pastilles are prepared using a low-shear jacketed blender and a pastillator. The resultant pastilles vary in shape and have a diameter of from about 4 to about 20 mm and a thickness of 2 to 4 mm.
Heterogeneous catalysts often include an active phase that is unstable in air. For example, highly reduced metal crystallites, such as cobalt crystallites having a 45% to about 90% reduction, are pyrophoric and are susceptible to oxidation. Further, if the catalyst is in the form of a reduced catalyst powder, the catalyst crystallite will have a particle size of from about 50 microns to about 150 microns. Thus, the catalysts can be difficult to handle and can create safety hazards when being loaded into a reactor.
In addition, the temperature rise from the internal section of the catalyst can cause oxidation of the active metal to an inactive metal oxide, which is undesirable for the reaction, and can cause sintering of the metal on the catalyst as well. Thus, it is advantageous to find a means for protecting the reduced metal catalyst. One common method is to form an oxide surface film by treating the reduced catalyst in a mixture of air and an inert gas. This procedure must be performed with extreme care because any surge of exotherm will cause sintering of the metal on the catalyst. Usually, the procedure starts with a very low oxygen concentration in a largely N2 stream and the oxygen concentration is gradually increased by increasing air/N2 ratio over a period of from about 72 to about 150 hours. An additional disadvantage of using this method for protecting the catalyst is that a fraction of the reduced metal is lost due to formation of metal oxide.
A more sophisticated method involves enrobing the reduced catalyst in a non-oxygen permeable media such as organic solvents, oils, fats and waxes. The enrobing or coating material works as an oxygen and moisture barrier to protect the metal being oxidized. By coating the catalyst, it is possible to stabilize the active material and to make handling the material easier. Further, the enrobing method allows essentially 100% of the reduced metal to be preserved.
The practice of coating or enrobing the active materials in a protective sheath is well known in the prior art. As early as 1952, a method for improving the coating of reduced nickel catalysts was taught in U.S. Pat. No. 2,609,346 (issued to Faulkner on Sep. 2, 1952). In the '346 patent, reduced nickel, usually containing a promoter, is dispersed in glyceride fat having a melting point from about 105° F. up to about 150° F. The mixture of catalyst and fat is melted at a temperature of about 160° F. to about 175° F., and then is cast in a metal form cooled to a temperature of between 50° F. and 60° F. to form a block of coated catalyst. Although this method produces an enrobed catalyst, the catalyst/fat mixture is formed into relatively large shapes that must be rapidly cooled to ensure that the wax is hardened throughout the block thereby preventing the catalyst from settling.
U.S. Pat. No. 2,842,504 (issued to Jones on Jul. 8, 1958) teaches a different method of coating a catalyst. In the '504 patent, a nickel-kieselguhr hydrogenation catalyst is coated with a rubbery polymer. The catalyst is added to a polymer/organic solvent solution and a rubber coat is formed on the catalyst by driving the organic solvent off. U.S. Pat. No. 3,453,217 (issued to Kozlowski et al on Jul. 1, 1969) describes a method of treating a catalyst with a liquid hydrocarbon having a boiling point in the range of 410° F. to 1200° F. The hydrocarbon is applied to the catalyst by discharging the catalyst into a container containing the liquid hydrocarbon and then moving the catalyst out of the container on a moving belt screen. If the process is carried out properly, the hydrocarbon fills the micropores of the catalyst. A somewhat different approach is taught in U.S. Pat. No. 6,294,498 (issued to Darcissac et al on Sep. 25, 2001). In the '498 patent, a catalyst is coated with a protective layer that is “atomized or dispersed on the catalyst by continuously stirring the catalyst and keeping it at a temperature that is below the crystallization point of the coating material”. Alternatively, the coating material may be in a solution that is “atomized, sprayed or dispersed by continuously stirring the catalyst at a temperature that is above the boiling point of the solvent of said solution.” Each of these methods result in the application of a protective coating on a catalyst. However, these methods either require specialized coatings or relatively sophisticated handling to ensure that the coating is deposited as intended.